A present invention relates to a method and apparatus for processing wafers. The invention has particular application to ion implantation chambers for semiconductor wafers.
In such ion implantation chambers, a wafer is scanned across an ion beam to introduce controlled doses of impurities into the wafer. The chamber in which the wafer is processed is evacuated.
In order to load the wafers into the vacuum chamber, a loadlock chamber is used to preserve the vacuum while loading wafers from the outside atmosphere. The loadlock chamber has an external valve to seal the loadlock chamber from the external atmosphere and an internal valve to seal the loadlock chamber from the vacuum chamber. With the internal valve closed and the external valve open, the wafer is loaded into the loadlock chamber from the atmospheric side. The external valve is then closed and the loadlock chamber is evacuated before the internal valve is opened and the wafer is transported into the vacuum chamber for processing. An example of such a loadlock is disclosed in EP-A-604,066.
In order to make most efficient use of the ion beam, and thus increase the throughput of the apparatus, the loading and unloading of the wafers into and out of the vacuum chamber must be done as quickly as possible. The present invention aims to improve the performance of the apparatus in this respect.
According to the present invention, an apparatus for processing wafers comprises a vacuum chamber in which the wafers are serially processed at a wafer processing position, two loadlocks through which the wafers are loaded into the vacuum chamber, and a mechanism for transporting the wafers from the loadlocks to the wafer processing position, each loadlock having an outer valve which is selectively operable to seal the loadlock from the external atmosphere, an inner valve which is selectively operable to seal the loadlock from the vacuum chamber, and a port for evacuation and pressurisation of the loadlock, wherein one loadlock is positioned above the other.
The use of two loadlocks which are preferably single wafer loadlocks allows wafers to be transported in parallel through the two loadlocks. Preferably a gripper arm is provided which is rotatable about an axis to access the loadlocks, and both loadlocks are positioned at the same radial distance from this axis. This allows the mechanisms for loading and unloading both loadlocks on one side of the loadlocks to share certain common parts. Preferably, the loadlocks are positioned one substantially directly above the other to allow this to be achieved with little or no increase in the footprint of the apparatus.
The internal and external valves of each loadlock can be slit valves in which a gate member is raised and lowered to uncover a slit allowing access to the loadlock chamber. Indeed, such valves are preferred for the external valves. However, preferably, each loadlock is a two-part structure, the two parts being movable together in a direction substantially perpendicular to the plane of the wafer to seal and provide the inner valve, and being movable apart in the opposite direction to break the seal and allow access to the wafer by a gripper arm pivoted about a single axis substantially perpendicular to the plane of the wafer.
To provide a more compact structure, it is preferred that the upper loadlock has an inner valve which is operable by upward movement of a top part, and the lower loadlock has an inner valve which is operable by downward movement of a lower part.
The wafers from both loadlock chambers can be picked up and set down by a robot which requires only axial motion in the direction of the axis about which the gripper arm is pivoted, and rotational motion about this axis. In fact, in order to allow a processed wafer to be loaded into the loadlock while an unprocessed wafer is being unloaded, a second gripper arm will be provided which is axially movable together with the first gripper arm. The second gripper arm is either disposed on the opposite side of the axis to the first gripper arm and is rotatable with the first gripper arm, or is positioned immediately below the first gripper arm and is rotatable about the axis independently of the first gripper arm. In the second case, which is preferred as it offers greater flexibility, the robot in the vacuum chamber is a three axis robot, having one linear and two rotational axes. This is advantageous over a conventional four axis robot as each additional axis required in a vacuum chamber increases the cost and the maintenance of the apparatus.
The robot forms an independent aspect of the present invention which can be broadly defined as a robot for transferring planar members from one location to another, the robot comprising a pair of coaxial shafts extending, in use, perpendicular to the plane of the planar members, each shaft having a gripper for gripping a planar member, the two shafts being rotatable independently about the axis and movable together along the axis, and a source of motive power for providing the axial and rotational motion.
The coaxial shafts also preferably contain air ducts for the supply of air to a pneumatic mechanism on each gripper arm for opening and closing the gripper arm. The coaxial shafts are preferably axially movable together by means of a lead screw.
The invention also extends to a method for loading wafers from an atmospheric source to a processing position within a vacuum chamber, the method comprising the steps of loading a wafer into a first loadlock chamber at atmospheric pressure, closing an external valve on the loadlock chamber and evacuating the loadlock chamber, opening an internal valve between the loadlock chamber and a vacuum chamber, transporting the wafer to the processing position, and repeating the process for a second loadlock chamber directly below the first loadlock chamber. Preferably access to the loadlocks requires a gripper arm rotatable about an axis, and both loadlocks are positioned at the same radial distance from this axis. The second loadlock chamber is preferably substantially directly below the first.
The transportation of the wafer to the processing position is preferably done by a robot having a gripper arm which is elevationally moveable along an axis which is substantially parallel to the direction in which the loadlock chambers are separated, and is rotatable about this axis, the method further comprising the steps of moving the gripper arm to a first elevational location aligned with the first loadlock chamber, rotating the gripper arm about the axis into the first loadlock chamber and picking up a wafer from the first loadlock chamber, rotating the gripper arm about the axis to transport the wafer to the processing position, depositing the wafer at the processing position, moving the gripper arm axially to a second elevational location aligned with the second loadlock chamber, and repeating the gripping, moving and depositing operations for a wafer from the second loadlock chamber.